V THE DEVELOPMENT OF THE FROG 103 



Organs from the Ectoderm. — In addition to forming 

 the outer layer of the skin over the entire surface of the 

 embryo the ectoderm gives rise to certain other structures 

 which come to He within the body. Chief among these is 

 the central nervous system whose beginning in the medul- 

 lary groove has already been described. The neural tube into 

 which the medullary groove develops loses its original con- 

 nection with the surface ; anteriorly it becomes enlarged 

 and forms the brain, the remaining portions developing into 

 the spinal cord. The thickening of the walls of the portion 

 of the tube which forms the cord diminishes the central 

 cavity until it becomes reduced to a fine canal, known in 

 the adult as the canalis centralis. The anterior portion of 

 the tube becomes divided by slight constrictions into three 

 vesicles, which form, designating them from before back- 

 ward, the fore, mid, and hind brain. The hindbrain 

 becomes widened from side to side, especially in front ; its 

 floor and sides thicken, but the roof, except for a small fold 

 at the end wliich develops into the cerebellum, remains 

 thin and membranous, and becomes thrown into a series of 

 folds which support a mass of blood vessels known as the 

 choroid plexus. The portion of the hindbrain which does 

 not form the cerebellum is converted into the medulla. 

 The central cavity becomes widened out, forming \hQ fourth 

 ventricle, which communicates posteriorly with the canalis 

 centralis of the cord and anteriorly with the ventricle of the 

 midbrain. 



The midbrain grows out dorsally and laterally into a pair 

 of hollow processes, the optic lobes, whose cavities or ventri- 

 cles communicate with the median canal, which becomes 

 narrowed by the thickening of its walls, and forms the aque- 

 duct of Sylvius, or iter a tertio ad quartum ventriculum. The 

 floor of the midbrain forms the crura cerebri. 



